Wavelength division multiplexing (WDM) is a technique which substantially increases the capacity of existing fiber optic networks. In a WDM system, plural optical channels are carried over a single waveguide, each channel being assigned a particular wavelength. Through time division multiplexing (TDM), each optical channel can carry the same amount of information as a conventional, single-channel optical system. Using optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical networks.
In order to monitor the operation of the optical network, it is desirable to know the signal levels and the optical noise at each optical channel wavelength. Because optical receivers are limited in sensitivity, the optical channel must have a sufficiently high signal-to-noise threshold to correctly distinguish the information contained on an optical channel within an acceptable error margin. Although the power levels of individual channels are easily measured at the end terminals in WDM optical systems, such power levels by themselves do not adequately relate to system performance since they fail to consider optical noise originating from optical amplifiers (amplified spontaneous emission-ASE) and other sources. Thus, there is a need in the art for improved optical monitoring systems which can measure optical power and generate signal-to-noise information for optical signals. Such optical monitoring systems could be used to produce signal-to noise information for each optical channel in a wavelength division multiplexed optical communication system. Further, there is a need in the art for determining optical channel signal-to-noise ratios at positions throughout the optical system, not merely at the end nodes. Such information could be used to present an accurate representation of overall system function and to aid in determining the location of equipment not performing to system specifications.